This invention concerns a method of producing magnesium hydroxide by reacting lime (CaO) with brine containing Mg.sup.++ ions and over 1 ppm boron (on a B.sub.2 O.sub.3 basis) wherein an excess of lime is reacted with the brine in a first reaction stage, and the high lime magnesium hydroxide is reacted with further brine in a second reaction stage.
Although there are many uses for magnesium hydroxide, one important one is as a raw material for producing refractory grain containing over 90%, and preferably over 95%, MgO. The production of such periclase refractory grain from magnesium hydroxide produced by reacting a brine containing magnesium ion with active lime has been practiced for many years (see Br. Pat. No. 1,502,422), and it is well-known that the properties of the refractory grain are highly dependent on the type and amount of impurities in the material.
For example, it is known that a refractory grain, and resulting refractory products such as brick, have greater strength when the boron content of the grain is low, for example less than 0.1%, and as low as 0.03% or lower. In this connection, U.S. Pat. No. 3,425,804 sets forth a method of achieving a low boron content magnesium hydroxide.
It is also known that the amounts of lime (CaO) and silica (SiO.sub.2), and particularly their relative amounts, influence the properties, particularly the high temperature strength, of periclase refractory grain. Specifically, grain having approximately a 2 to 1 weight ratio of lime to silica shows increased high temperature strength. Therefore, it is known to attempt to control the amount of lime in magnesium hydroxide produced as precursor to periclase refractory grain; see British Pat. No. 1,413,871.
One of the important properties of refractory grain is its density. It is desirable to have a periclase grain with a bulk specific gravity (BSG) of at least 3.35, and preferably of at least 3.40. Again, it has been found that the relative amounts of the minor impurities in magnesium hydroxide used to produce periclase grain greatly affect the sintered density of the grain.
Although it is possible to reduce the boron content of periclase refractory grain by firing it at very high temperatures (e.g., 1800.degree. C. or higher), it has been found that to achieve the highest BSG in such grain, it is essential that the magnesium hydroxide from which it is made be of low boron content. In other words, although it is possible to take a magnesium hydroxide containing, for example, 0.3% B.sub.2 O.sub.3 on the ignited basis, and fire this at 1800.degree. C. to produce a periclase grain, reducing the boron content to as low as 0.03% B.sub.2 O.sub.3, the BSG of the grain will be considerably lower (e.g., 3.25 as opposed to 3.4 or greater) than that of a grain similarly processed but using a magnesium hydroxide which contains only 0.05% B.sub.2 O.sub.3 to begin with. Thus, not only is it desirable to have a low boron content in the fired grain itself, but it is necessary to have a low boron content in the initial magnesium hydroxide in order to achieve the highest bulk specific gravity.
It has been known for some time that it is possible to reduce the amount of boron in a magnesium hydroxide produced from a boron-containing brine by precipitation with active lime if an excess amount of lime is used in the precipitation. However, in previous processes using this method, the resulting magnesium hydroxide has a higher lime content than is most desirable. Although such "high lime" magnesium hydroxides (i.e., magnesium hydroxide containing over 1% CaO, and specifically having a lime to silica ratio of 2 or greater) sinter rather readily, there are many applications where a lower lime content is desired.
According to the process of this invention, it is possible to produce a magnesium hydroxide which contains, on the ignited basis, less than 1% CaO and less than 0.1% B.sub.2 O.sub.3, and which will sinter to a BSG of over 3.40.